The present invention relates to a method for designing a diffractive lens to be formed on a surface of a refractive lens, and more particularly, to a generalized method for designing a diffractive-refractive hybrid lens on which rays may be incident obliquely with respect to an optical axes of the lenses.
There is conventionally known a lens formed with a circular zone structure, which functions as a diffractive lens, on a curved surface of a refractive lens in order to compensate for chromatic aberration with a single lens element. An example of a lens having such a structure is disclosed in Japanese Patent Provisional Publication No. HEI 6-242373, which discloses an objective lens for optical disk drive, and the diffractive lens structure described as above is formed on a surface thereof.
Further, Japanese Patent Provisional Publication No. HEI 8-171052 discloses a designing method employing a high refractive index method utilizing a Sweatt model. Then, based on the designed results, a diffractive lens is replaced with a thin film having a very high refractive index, and the lens is designed. According to this method, locations at which steps of circular zones to be formed, and an amount of the steps are determined. Further, sag amounts of the zones are determined as a sum of sag amounts of a surface of a refractive lens on which the diffractive lens is formed and sag amounts of the diffractive lens structure.
The objective lens disclosed in the Japanese Patent Provisional Publication No. HEI 6-242373 requires complicated calculations since aspherical surface coefficients are determined for each zone. The calculation disclosed in the Japanese Patent Provisional Publication No. HEI 8-171052 defines variations of a sag amount within one zone of a diffractive lens structure using an expression below: EQU s(z)-(r(z).sup.2 -H.sup.2).sup.1/2
Since the above expression is an expression representing a spherical surface, this expression involves the following deficiencies.
(1) the expression involves a relatively large error if the diffractive lens structure includes a portion whose power is close to 0, i.e., a radius r of curvature of the portion is very great; and
(2) since the structure is expressed with a quadratic function, only a limited optical performance of the diffractive lens is obtained.
Therefore, a lens structure obtained by the method may produce greater aberrations in comparison to the diffractive lens structure obtained by the method disclosed in the Japanese Patent Provisional Publication No. HEI 6-242373.